Help me please??? MARIO KART DS HELP!!?

I need help on Mario Cart Ds. I just got my Nintendo DS today and played Mario Cart on wifi. How do I get turbo speed at begining of race? When it counts down to 3....2....1 then goes? Please? I don't get it. I hold on to A when 3..2..1 0 goes n my car explodes everyone zooms ahead of me. How do use turbo speed at begining when countdown starts please?